In the statement, the bin Laden family members called on the International Criminal Court, the International Court of Justice, and the United Nations to provide answers, and threatened legal action if their request was ignored. The letter notes that just as Omar publicly criticized his father's use of violence, so too would he and his brothers now "condemn the president of the United States for ordering the execution of unarmed men and women."

The bin Laden family also criticized the US for firing on unarmed people in the compound during the raid and for burying Osama bin Laden at sea instead of notifying the family to conduct a proper burial. The letter called on Pakistan to release all the family members in custody.

His comments echoed US Attorney General Eric Holder, who said in a Senate hearing last week that because bin Laden was considered an enemy commander, targeting him was lawful. Days after 9/11, Congress passed the Authorization for Use of Military Force Act allowing the US president to use "all necessary and appropriate force" against anyone who "planned, authorized, committed, or aided" in the execution of the 9/11 attacks.

The international organization Human Rights First said the White House, and not Omar bin Laden, appeared to be on firm legal footing. "Assuming the existence of an armed conflict against Al Qaeda, Osama bin Laden was targetable unless he was surrendering or so injured as to no longer be apparently capable of engaging in hostilities," it said in a statement.

Amid contradicting accounts of the Navy SEAL raid, however, it is unclear to what extent bin Laden resisted. In the initial account, bin Laden was armed – a claim the US later retracted. Since then, the US has maintained that bin Laden resisted, but did not provide more details.

It is "questionable whether the USA can still claim to be engaged in an armed conflict with Al Qaeda," Professor Kress told Der Spiegel.

According to Der Spiegel, the laws of war "permit the targeted killing of non-state combatants, provided they are really combatants who are organized in units with a military-like character, and that they are integrated into those units either as armed fighters or as a leader who issues commands."